1. Field of the Invention
This invention relates to a method of depicting a desired pattern of elements with an electron beam on a resist layer formed on a disc-like substrate, a disc-like substrate for a high-density recording such as a master information carrier for magnetic transfer, an optical disc stamper and a patterned medium for high-density magnetic recording and a recording medium.
2. Description of the Related Art
There has been known magnetic transfer where the surface of a master information carrier having thereon a fine irregularity pattern of magnetic material representing information to be transferred is brought into close contact with a surface of a slave medium (a magnetic recording medium) having a magnetic layer to which the information is transferred and a transfer magnetic field is applied to the slave medium and the master information carrier in this state, thereby recording on the slave medium a magnetization pattern representing the information (e.g., a servo signal) on the master information carrier. See, for instance, Japanese Unexamined Patent Publication No. 63(1988)-183623 and U.S. Pat. No. 6,347,016.
The master information carrier generally comprises a substrate and an irregularity pattern (a pattern of protruding portions and recessed portions) of magnetic material formed on the substrate.
It has been expected that the master information carrier for magnetic transfer can be produced by the use of a method of producing an optical disc stamper, for producing optical discs, on the basis of a matrix carrying thereon an irregularity pattern of resist representing information to be transferred. (See, for instance, in U.S. Patent Laid-Open No. 20010028964.)
Further, it is conceivable that the irregularity pattern of resist corresponding to the irregularity pattern on the substrate of the master information carrier for magnetic transfer can be depicted on the resist layer formed on a disc-like matrix by exposing the resist layer to a laser beam modulated according to information to be transferred while rotating the matrix in the same manner as the production of the optical disc matrix.
However, as the track width is narrowed (e.g., to not larger than 0.3 μm), for instance, to meet a demand for a higher recording density, the drawing diameter of a laser beam (the diameter at which a laser beam draws an image on the resist layer) comes not to be able to be thinned to draw a pattern of lands (protruding portions) and grooves (recessed portions) in such a narrow tracks. As a result, elements of the irregularity pattern of resist come to have arcuate end portions and cannot be rectangular in shape. The element of the irregularity pattern on the master information carrier conforms to the element of the irregularity pattern of resist on the matrix in shape, especially in shape of the upper surface of the element. Accordingly, when the element of the irregularity pattern of resist on the matrix has end portions which are arcuate and not rectangular, the element of the irregularity pattern on the master information carrier also has end portions which are arcuate and not rectangular. Arcuate end portions of the lands of the irregularity pattern on the master information carrier result in incorrect formation of a magnetization pattern on the slave medium.
We, this applicant has proposed, in Japanese Patent Application 2002-202629, a method of depicting a pattern on the resist layer of the matrix by the use of an electron beam which is smaller in beam diameter than the laser beam.
In the method disclosed in Japanese Patent Application 2002-202629, an irregularity pattern is depicted on the resist layer with an electron beam whose diameter is smaller than the minimum width of the elements (such as the upper surfaces of the protruding portions or the openings of the recessed portions) of the irregularity pattern, and the shape of each element is drawn by causing the electron beam to scan the resist layer a plurality of times. For example, in the case where an element of the irregularity pattern is of a rectangle perpendicular to the direction of recording tracks (the circumferential direction), the matrix is slightly rotated each time the electron beam scans the matrix in the direction perpendicular to the direction of recording tracks. The one scanning of the electron beam and the fine rotation of the matrix are alternately repeated a plurality of times, thereby depicting one element.
However, the method involving causing an electron beam to scan in radial directions a plurality of times is disadvantageous in that, when the irregularity pattern includes an element which is a parallelogram having a slant side obliquely intersecting the direction of recording tracks in its shape as well as an element the shape of which is of a rectangle perpendicular to the direction of recording tracks extending vertically to the same (e.g., a phase servo pattern), the slant side is zigzagged and cannot be precisely formed. The slant side of the element which is a parallelogram in shape corresponds to a magnetization transition zone, and the linearity thereof is very important upon reproduction of the signal. Further, it is required to precisely control the inclination of the slant side to the recording tracks.
It is conceivable to meet demands described above by a method in which elements are depicted by causing an electron beam to scan back and forth a plurality of times in a direction of the slant side. However, these inventors have found that even if the electron beam is caused to scan to trace a trajectory inclined by the same angle as the desired inclination of the slant side, a slant side inclined by the desired angle cannot be actually obtained since the area exposed to an electron beam depends upon overlapping of the trajectory of the electron beam, the exposure time and/or the like and overlapping of the trajectory of the electron beam differs according to the part where the electron beam overlaps, near the peak or the valley or at an intermediate portion.
Accordingly, there has been a demand for a method which can easily depict an element having a desired slant side with an electron beam.
Also in the field of optical discs, it will become necessary to depict a pattern with an electron beam upon making a stamper in order to obtain a higher recording density.
It has been proposed to depict a pattern with an electron beam in production of a patterned medium realization of which has been expected as a high density magnetic recording medium which can be small in size and light in weight. See, for instance, Japanese Unexamined Patent Publication No. 2001-110050. However, in Japanese Unexamined Patent Publication No. 2001-110050, though use of an electron beam has been disclosed, how to depict a pattern with an electron beam has not been disclosed in detail.
With an increase in information quantity, there is a demand for a magnetic recording medium which is high in memory capacity, low in cost and preferably requires a short time to read out a necessary part of data (a magnetic recording medium which allows so-called high-speed access). As an example of such a magnetic recording medium, there has been known a high recording density magnetic medium such as a hard disc, a zip (Iomega) and the like. In such a high recording density magnetic medium, the recording area is formed by narrow data tracks. In order to cause a magnetic head to accurately trace such narrow data tracks and reproduce the data at a high S/N ratio, the so-called servo tracking technique has been employed.
In order to perform the servo tracking, it is necessary to write servo information such as servo tracking signals for positioning the data tracks, address signals for the data tracks and reproduction clock signals on the magnetic recording medium as a preformat upon production thereof. At the present, such preformat recording is performed by the use of a specialized servo recording apparatus (a servo track writer). However, the preformat recording by the conventional servo recording apparatus is disadvantageous in that it takes a long time since the servo information must be recorded on the magnetic recording medium one by one by the use of a magnetic head, which deteriorates the productivity.
As a method of recording the preformat accurately and efficiently, there has been proposed, for instance, in Japanese Unexamined Patent Publication No. 63(1988)-183623 and U.S. Pat. No. 6,347,016, a magnetic transfer method in which a pattern which is formed on a master information carrier and represents servo information is copied to a magnetic recording medium (a slave medium) by magnetic transfer.
In the magnetic transfer, the magnetization pattern representing the information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other, and accordingly, the information carried by the master information carrier can be statically recorded on the slave medium with the relative position between the master information carrier and the slave medium kept constant. Thus, according to the magnetic transfer, the preformat recording can be performed accurately and the time required for the preformat recording is very short.
The conventional servo writing by the use of a servo track writer can suppress the azimuth loss since it records the servo signal by the use of the same magnetic head. However, when a signal represented by a magnetization pattern of a magnetic disc on which the magnetization pattern is formed by the magnetic transfer employing the master information carrier is to be reproduced by a magnetic head of a recording and reproducing system, the azimuth loss can become large since recording and reproduction are effected in quite different systems. Increase in the azimuth loss results in deterioration of the quality of the reproduced signal. Especially, when the transferred signal is a servo signal, increase in the azimuth loss results in deterioration of accuracy of the tracking servo.
Production of any one of the master information carrier, the optical disc stamper and the patterned medium involves depicting a desired pattern of elements on a resist layer formed on a disc-like substrate, and forming an irregularity pattern corresponding to the desired pattern of elements.